The use of silicon in liquid crystals is known in principle but it is, in particular, polymeric liquid crystals having poly- or oligosiloxane backbone in linear or cyclic arrangement with different mesogenic side chains which are described, for example, by H. Finkelmann and G. Rehage in Macromol. Chem. Rapid Commun. 1, 31, (1980), or else in EP-A 29 162 (Finkelmann et al.; laid open for public inspection on May 27, 1981, Wacker-Chemie GmbH). Cyclosiloxanes with mesogenic side groups are disclosed in EP-A 60 335 (Kreuzer et al.; laid open for public inspection on Sept. 22, 1982, Consortium fur elektrochemische Industrie GmbH).
In addition, monomeric liquid crystals which use in the wing groups either trialkylsilyl radicals, as in DE-A 38 27 600 (Hemmerling et al.; laid open for public inspection on Feb. 15, 1990, Hoechst AG), or polymethyloligosiloxanes, as in JP 89/144491 (laid open for public inspection on Jun. 6, 1989, abstracted in Chemical Abstracts, Vol. 111, 205 667 e (1989), or EP-A 404 140 (Haas et al.; laid open for public inspection on Dec. 27, 1990, Consortium fur elektrochemische Industrie GmbH) are also known. Owing to these silanyl or siloxanyl radicals, the resulting properties are changed from those of the unsilylated compounds. This relates in particular to melting point depressions, phase broadening and changes in the types of phases. The trialkylsilyalkylcontaining liquid crystals however result in smaller modifications compared with unsilylated compounds. Although the siloxanylsubstituted derivatives have more of an effect in this sense, their chemical resistance is low and, specifically in the case of larger siloxanes, they can be prepared in pure form only with difficulty because higher siloxanes can often be obtained only in mixtures which are difficult to separate. The lack of chemical stability of the Si--O bonds means that the introduction of the siloxanyl radicals must be the last chemical reactions of the syntheses in order not to decompose the entire molecule again. However, this requirement cannot be met when, as frequently occurs in liquid crystals, hetero atoms such as nitrogen or sulfur are present, which prevent silylation or siloxanation.
Therefore, it is an object of the present invention to provide liquid crystals which are easier to prepare and more chemically resistant and have improved liquid crystalline properties such as wider phase ranges and lower melting points.